1. Field of the Invention
This invention relates to gain-controlled amplifiers in general and, more particularly, to gain-controlled amplifiers having cascaded gain stages.
2. Description of the Prior Art
Amplifiers having variable gain are used in a variety of applications, such as in instrumentation, where selectable and precise gain control may be needed. Another application is in a superheterodyne radio receiver, where most of the gain in the receiver is provided by an intermediate-frequency (IF) amplifier. For the receiver to be useful in practical applications, it is usually desirable to have the receiver successfully process signals with widely differing intensities. In other words, the receiver should be designed to handle the largest possible difference in signal strength between the weakest received signal that can be utilized and the largest received signal before overload occurs. This is referred to as the dynamic range of the receiver. To increase the dynamic range, the receiver's IF amplifier is usually adapted to be responsive to a means for controlling the gain so that weak signals are amplified sufficiently for intelligibility while strong signals do not overload subsequent circuitry. An example of a controlling means in a typical radio receiver is a peak detector coupled to the output of the IF amplifier. The peak detector samples the output of the IF amplifier to provide an analog gain control signal that is fed back to the IF amplifier. The stronger the received signal, the larger the output of the IF amplifier and the gain control signal varies accordingly to reduce the gain of the IF amplifier. Similarly, when the received signal is reduced, the output of the IF amplifier decreases and the gain control signal reacts accordingly to increase the gain of the IF amplifier. The variation in IF amplifier gain may be as large as 50 to 100 dB, increasing the receiver's dynamic range accordingly.
As known in the prior art, the generation of the control signal may be derived directly from the peak detector or may be generated from a digital control system, such as a microprocessor or a digital signal processor. To create the analog gain control signal from a digital one, a digital-to-analog converter (DAC) is used to convert the digital data from the microprocessor into an analog signal. This makes the receiver more expensive considering the inclusion of a DAC either as a separate device or as part of the IF amplifier or microprocessor. Further, the analog gain control signal does not have a linear correspondence to the gain; "kinks" or nonlinearities exist in the voltage-to-gain curves of the analog gain control signal for the IF amplifier. Further, the voltage-to-gain curves may vary with temperature. The "kinks" and other variations may have to be compensated for in the microprocessor software if precise gain control is to be achieved.
An alternative approach for microprocessor controlled receivers is having multiple gain stages, each with fixed gain, which are switched in or out of the signal path depending on the desired overall IF gain. This approach requires multiple switches, typically one per gain stage, each switch being as or more complex than the corresponding gain stage. This also increases the cost of the receiver by approximately doubling the cost of the IF amplifier therein.